The present invention relates to a coated cemented carbide insert (cutting tool) particularly useful for wet and dry milling of low and medium alloyed steels and stainless steels. It is also excellent for turning of stainless steels.
When machining low and medium alloyed steels and stainless steels with cemented carbide tools, the cutting edge is worn according to different wear mechanisms, such as chemical wear, abrasive wear, adhesive wear and by edge chipping caused by cracks formed along the cutting edge.
During milling, which is an intermittent cutting process, the cutting edge is exposed to thermal variations that cause the thermal cracks mentioned above. These cracks will finally destroy the cutting edge.
During turning, which can either be a continuous or an intermittent cutting process, the cutting edge is exposed to variations in cutting forces and thermal variations that cause the cracks mentioned above. These cracks will finally destroy the cutting edge.
Measures can be taken to improve the cutting performance with respect to a specific wear type. However, very often such action will have a negative effect on other wear properties. The following has generally been accepted:
Thermal crack formation may be reduced by lowering the binder phase content. This measure will, however, also reduce the toughness properties of the cutting insert which is generally not desirable,
The toughness may be improved by increasing the binder phase content. However, this measure will decrease the plastic deformation resistance and in general increase the abrasive wear and the formation of thermal cracks.
The deformation resistance may be increased by reducing the grain size of the carbide phase. However, this measure has a negative effect on the crack initiation and propagation which gives rise to edge chipping.
An alternative way to increase the deformation resistance is to add cubic carbides like TiC, TaC and/or NbC. This will, in general, also increase the wear resistance when machining at high cutting edge temperatures. However, this addition also has a negative influence on the formation of thermal cracks and edge chipping.
So far it has been very difficult to improve all tool properties simultaneously. Commercial cemented carbide grades have therefore been optimized with respect to one or few of the above mentioned wear types and consequently also to specific application areas.
WO 97/20083 discloses a coated cutting insert particularly useful for milling of low and medium alloyed steels and stainless steels with raw surfaces such as a cast skin, forged skin, hot or cold rolled skin or pre-machined surfaces under unstable conditions. The insert is characterized by a WC-Co cemented carbide with a low content of cubic carbides and a rather low W-alloyed binder phase and a coating including an innermost layer of TiCxNyOz with columnar grains and a top layer of TiN and an inner layer of xcexa-Al2O3.
WO 97/20081 describes a coated milling insert particularly useful for milling in low and medium alloyed steels with or without raw surface zones during wet or dry conditions. The insert is characterized by a WC-Co cemented carbide with a low content of cubic carbides and a highly W-alloyed binder phase and a coating including an inner layer of TiCxNyOz with columnar grains, an inner layer of xcexa-Al2O3 and, preferably, a top layer of TiN.
WO 97/20082 discloses a coated turning insert particularly useful for turning in stainless steel. The insert is characterized by a WC-Co-based cemented carbide substrate having a highly W-alloyed Co-binder phase and a coating including an inner layer of TiCxNyOz, with columnar grains followed by a layer of fine grained xcexa-Al2O3 and a top layer of TiN.
U.S. Pat. No. 5,700,569 discloses an alumina coated cemented carbide insert having improved properties for metal cutting applications. The insert has six to eight layers of alumina with a total coating thickness of up to about 15 xcexcm.
U.S. Pat. No. 4,984,940 discloses an indexable metal cutting insert having a cobalt cemented tungsten carbide substrate with a multi-layer refractory coating thereon. The substrate has a cobalt content of 6.1 to 6.5 weight percent. The coating contains at least a plurality of alumina layers which are separated from and bonded to each other by a group IVB metal nitride, such as titanium nitride, and which are bonded to the substrate by a backing layer of 5 to 8 xcexcm in thickness, composed of a carbide and/or carbonitride of titanium, zirconium and/or hafnium.
U.S. Pat. No. 6,015,614 discloses an Al2O3xe2x80x94TiN coated cemented carbide insert intended for turning of steels and especially Ca-treated steels. The alumina layer is protected by an extra thick and multilayered coating of TiN.
It has now been found that enhanced milling and turning performance can be obtained by combining the substrate and the multi-layer coating of the present invention. The cutting insert has excellent performance in low and medium alloyed steel but particularly in stainless steel. The cutting tool displays an improved behavior with respect to many of the wear types mentioned earlier, in particular to formation of edge chipping caused cracks along the cutting edge.
According to one aspect, the present invention provides a cutting tool insert comprising a cemented carbide body and a coating, said coating including a multi-layer coating with a total thickness varying from 2 xcexcm to 20 xcexcm, said coating being composed of xcexa-Al2O3 layers with a thickness of 0.1-0.4 xcexcm, and TiN or Ti(C,N) layers with a thickness of 0.3 to 0.6 xcexcm; and that said cemented carbide body comprising WC with a mean intercept length of 0.5-0.9 xcexcm, 9.0-10.9 wt-% Co and 0.5-2.5 wt-% TaC+NbC with a ratio of the weight concentrations of Ta and Nb 7.0-12.0, and a binder phase with an S-value of 0.81-0.92.